PLCs (programmable logic controllers) that control control target devices (such as industrial machines) used in factories and the like include device memories for temporarily storing respective pieces of device data on the control target devices. Such PLCs (sequencers) control the control target devices by reading and writing the respective pieces of device data in accordance with a sequence program that is a program created by the user. A technology for use in systems in which the PLC is used is widely known. With this technology, the values of respective pieces of device data are transmitted to the PC (personal computer) side and the changes and the state of each device are displayed on the display apparatus such as a PC.
With the above conventional system in which the PLC is used, when there is a transmission request from the PC to transmit device data to the PLC, all the device data acquired by the PLC at specified time intervals is transmitted to the PC. In such a case, as the amount of device data requested by the PC side to be transmitted increases, the amount of data to be transmitted from the PLC to the PC also increases. Thus, the load on the device-data communication process increases, which in some cases results in the loss of device data to be transmitted to the PC or affecting the processes of other functions that are controlled by the PLC.
In such a system, there is device data whose value did not change from that of the previously transmitted device data among the values of device data to be transmitted. It is wasteful, for the system that displays the state of data, to transmit device data whose value did not change and this causes an increase in the communication load. Thus, to reduce the communication load, a method has been used in which the values of the previously transmitted device data are compared with those of the currently transmitted device data and only the device data whose value has changed is transmitted (for example, see Patent Literatures 1, 2, and 3).